virtio-blk: fix implicit overflow on virtio_max_dma_size

In the Linux kernel, the following vulnerability has been resolved: virtio-blk: fix implicit overflow on virtio_max_dma_size The following codes have an implicit conversion from size_t to u32: (u32)max_size = (size_t)virtio_max_dma_size(vdev); This may lead overflow, Ex (size_t)4G -> (u32)0. Once virtio_max_dma_size() has a larger size than U32_MAX, use U32_MAX instead.